1. Field of the Invention
The present invention relates to syringe pumps which discharge a small quantity of liquid using a syringe which includes a cylindrical casing and a piston enclosed therein and used as a liquid discharge unit in a medical device or an industrial device.
2. Description of the Related Art
Recently, demands for discharging a small quantity of liquid arise in various fields of industries, for example, a demand for administering a carcinostatic substance dropwise in the field of medical cares over a long time and a demand for discharging a minimum required quantity of soldering paste to a welded point in the field of electronic part manufacture. Generally, a liquid such as a carcinostatic substance to be discharged is exchanged in accordance with application and easy exchange of such liquid is required. In the past, a syringe pump is used to discharge a liquid accurately and to facilitate the exchange of that liquid.
FIG. 12 shows a technique related to syringe pumps. Disposed within a box-like frame 50 in FIG. 12 is a guide rod 51 to which a member 52 movable axially of guide rod 51 is attached. Cylindrical casing 71 of syringe 70 is fixed on top of frame 50 by syringe holder 53. Syringe piston 72 is engaged with movable member 52, so that when piston 72 moves forward in accordance with the movement of movable member 52, a liquid is discharged out of cylinder 70. Screw shaft 54 is provided rotatable and parallel to guide rod 51 within frame 50. A pair of half nuts 55 is engaged on screw shaft 54. Upper half nut 55 is connected to rod 56 attached to movable member 52. Drive gear 57 is attached to an end of screw shaft 54 and connected to gear 60 fixed to output shaft 59A of motor 59 through intermediate gear 58. Therefore, when motor 59 is driven, screw shaft 54 is rotated through gears 57, 58 and 60, so that half nuts 55 and movable member 52 are moved axially. As shown in FIG. 13, intermediate gear 58 is attached rotatably to one end of swingable arm 61 the other end of which is attached swingably to frame 50. A detection element 62A of switch 62 abuts on swingable arm 61. Spring 63 is attached to the free end of arm 61 to which intermediate gear 58 is attached so as to bias intermediate gear 58 against gears 57 and 60. Spring 63, switch 62, arm 61 and intermediate gear 58 constitute closure detecting mechanism 64 which detects closure prescribed in JIS (Japanese Industrial Standard) (6.4 in T1635-1986). If syringe 70 is closed for some reason, the movement of movable member 52 is hindered in spite of the operation of motor 59. If closure pressure exceeds an allowable value, a slippage occurs between intermediate gear 58 and gear 60 attached to motor 59, so that intermediate gear 58 moves away from gears 57, 60 against the action of spring 63. Switch 62 detects such movement of intermediate gear 58 and generates a signal which causes a controller (not shown) to stop motor 59.
In order to allow syringe 70 to be replaced, piston 72 must manually be moved freely relative to casing 71. To this end, the syringe pump has a clutch mechanism which transmits the drive force of screw shaft 54 to movable member 52 or releases the transmission of the drive force of screw shaft 54 to movable member 52. In the syringe pump of FIG. 12, the clutch mechanism has a structure in which the pair of half nuts 55 abuts disengageably on screw shaft 54.
Another technique related to a syringe pump is disclosed in Published Unexamined Japanese Utility Model Application Sho 63-190921 in which the syringe pump includes a frame having a mount for a syringe, a lever supported by the frame and fixing the syringe to the mount, a movable member supported movably on the frame for operating the piston in the syringe, a motor attached to the frame, a roller provided on each of an output shaft of the motor, the movable member and the frame, and a wire belt extending around these rollers to move the movable member in accordance with the drive of the motor. If the motor is driven, the wire belt causes the movable member to be driven on the principle of a running block to discharge a liquid from the syringe. The clutch mechanism is attached to the lever such that when the lever is operated, the wire belt is rendered tensile or released to transmit the drive force of the motor to the movable member or render the movable member freely movable.
However, in the syringe pump of FIG. 12, the screw shaft and half nuts each are a precise mechanical part to thereby render the syringe pump expensive, disadvantageously. In order to adjust a replaced syringe minutely, the piston can be moved by an appropriate stroke with the syringe casing being fixed, in which case the half nuts 55 are at any position along screw shaft 54 under which condition the clutch is actuated. Since the position of half nuts 55 relative to screw shaft 54 is not fixed, half nuts 55 and screw shaft 54 may not coincide in thread crest in which case half nuts 55 and screw shaft 54 are not appropriately engaged with each other and can rotate uselessly. Therefore, the position of movable member 52 must manually be adjusted minutely such that half nuts 55 and screw shaft 54 are appropriately engaged with each other. This adjustment is troublesome. Even if half nuts 55 and screw shaft 54 coincide with each other in thread crest and bottom, there may be a backlash between the crests and bottoms, so that there occurs an error between a quantity of drive given by motor 59 and a quantity of movement of movable member 52 and hence an error in a discharged quantity of liquid.
Since the syringe pump of the Application '921 uses the wire belt extending around the roller provided on the motor output shaft to transmit the drive force by the frictional force occurring between the roller and wire belt, there arises a slippage between the wire belt and roller to thereby hinder accurate discharge of liquid disadvantageously. Especially, if the roller rapidly rotates at high speed directly after the actuation of the motor, the roller becomes likely to slip relative to the wire belt due to transition from static friction to dynamic friction. In order to eliminate such slippage, the wire belt is required to be extend around the rollers doubly or triply. In this case, the wire belt would be rubbed by itself and deteriorated in durability.